Review/Film; Fletch Goes South

Michael Ritchie's ''Fletch Lives'' is the overly optimistic title of the follow-up to the far more entertaining 1985 Ritchie comedy ''Fletch,'' about a wise-cracking investigative reporter played by Chevy Chase. ''Fletch Lives'' looks less like ''Fletch 2,'' which it is, numerically speaking, than ''Fletch 7,'' the bitter end of a worn-out series.

Mr. Chase is such an agreeably low-pressure comedian that a movie has to be very inept to be as irritating as ''Fletch Lives.'' The film, opening today at the Manhattan Twin and other theaters, takes Fletch to the Deep South when he inherits a run-down plantation. In this way, he and the movie makers place themselves up to their heels in silly jokes about the Ku Klux Klan, toxic waste and television evangelists.

Movies as bad as this have a way of announcing themselves as soon as they can. Early in the narrative, when Fletch is en route to the South, he dreams up a big production number in which he and what look to be 1,000 extras sing ''Zip-a-Dee-Doo-Dah'' and dance around on immaculate lawns, under trees laden with Spanish moss.

Suddenly the singing and dancing just stop, as if the man editing the movie had also gotten bored and switched to another channel.

The rest of the movie lurches along in the same fashion, without point or good feeling.